A(1). Field of the Invention
The invention relates to an image enhancement circuit. Such circuits are used to produce a high quality, image from a plurality of quasi-stationary low quality images. For example, let it be assumed that a high-quality TV image is desired of an object which is at a long distance from a TV camera. When the atmosphere is quiet (as with light frost), a high quality image can generally be obtained without image enhancement. However, if the atmosphere is very turbulent (as on very hot days) a low quality is generally produced, with the image being highly degraded by noise. To obtain an image of high quality under such circumstances, a plurality of low quality images of this object is processed in the image enhancement circuit.
Similar situations occur in scanning electron microscopy, X-ray fluoroscopy and in astronomy.
A(2). Description of the Prior Art
A known method of obtaining an image of high quality from a series of low-quality images is described on page 434 of a book by W. K. Pratt entitled Digital Image Processing (John Wiley and Sons, 1978). In this method, the average value is of the series of low-quality images is calculated. This may lead to the desired result, which is evident as follows. For the i-th low-quality image G.sub.i (x,y), EQU G.sub.i (x,y)=H(x,y)+N.sub.i (x,y) (1)
In this equation H(x,y) represents the desired high-quality image and N.sub.i (x,y) represents an additive noise image which is independent of the high-quality image. When the desired high-quality image is constant for a series of M low-quality images, ##EQU1## For a high value of M, the noise term on the right-hand side approximates zero under normal circumstances, so that: ##EQU2##
A device for performing the above-mentioned method will generally comprise a circuit for scanning the low-quality images G.sub.i (x,y) in an image scan period T.sub.b, for generating low-quality image signals g.sub.i (t). These signals are weighted in a weighting circuit with a weighting factor 1/M. The low-quality image signals thus weighted are accumulated in an accumulator. After all M low-quality image signals have been applied to this accumulator, the supply of further low-quality image signals is impeded. At that moment, the output of the accumulator supplies a high-quality image signal f(t) which represents the desired high-quality image H(x,y). The high-quality image can be displayed by applying f(t) to a monitor.
As has been described on page 686 of an article by H. Bruggemann entitled "Temporal Filtering Using Pixel Incrementing" (Journal of the Society of Motion Picture and Television Engineers, pages 686-694 (August, 1981), a drawback of this known image enhancement circuit is that the desired brightness of the image cannot be obtained until after all M weighted low-quality image signals are accumulated. A further drawback of this known circuit is that the optimum value of M is not always known in advance. In fact, this will be greatly dependent on the circumstances.